LONDON, Sept 18 (Reuters) - European shares fell on Friday after the U.S. Federal Reserve kept interest rates unchanged, fuelling concerns over the global economy and leaving investors guessing about when a policy tightening will start.

The Fed kept rates at zero in a bow to worries about the global economy, financial market volatility and sluggish inflation at home, but left open the possibility of a modest policy tightening later this year.

Traders said uncertainty over when the Fed would eventually raise rates was weighing on stock markets, along with the Fed's comments about pressures caused by signs of a slowdown in China.

"It's the uncertainty over the state of the economy, and a sense of unfinished business over when the Fed will make its move, that is weighing on markets," said Mirabaud Securities' senior equity sales trader John Plassard.

Banks, which often make more money in a higher interest rate environment, were among the worst performers while exporters such as carmakers and luxury good stocks - for whom China is a key market - also lost ground.

French payments company Ingenico Group rose 7.5 percent as prospects it could stretch its finances to launch a bid for Worldpay evaporated on Friday when its privately-owned British rival announced plans for a London floatation, instead of a sale.

Germany's Wirecard, also seen in the running for Worldpay, rose as much as 1.7 percent before paring gains.

Shares in Hella, a German manufacturer of auto headlights and electronics, slumped 8.6 percent after issuing a profit warning due to problems in China.

In spite of Friday's pull-back, some investors and analysts were still backing European shares, arguing that the region's equity markets would be supported by economic stimulus measures from the European Central Bank.

Lex Van Dam, hedge fund manager at Hampstead Capital, said equities remained his preferred asset class since returns on bonds and cash were still depressed by the low interest rates set by major world central banks.

Goldman Sachs Asset Management, however, warned of more market volatility given uncertainty over when the Fed would eventually raise rates.

"We believe an October rate hike is unlikely and the December meeting is a toss-up. We think this delay could dampen market volatility in the near term, though uncertainty about Fed policy could lead to more volatility as the December meeting approaches," it wrote in a note to clients. (Additional reporting by Danilo Masoni in Milan; Editing by Ruth Pitchford)